Nowadays, as the application of network technology becomes popular, people can share data with friends over the ubiquitous networks. Take the web photo album for example, one can upload photos to be shared with others to a network server and anyone who is interested in the photos can appoint and see the photos. To see a photo, the image display system may need to transmit, decode, and display the photo file. The approaches for displaying photos in the web photo album can be displaying a photo that is individually appointed one at a time or displaying photos that are appointed as a group one after another. When the photos on the network server are sequentially displayed, observers can view the pictures more smoothly if the photos are displayed at an interval of a constant period such as every 5 seconds. However, it is quite frequent that the display delays for the reason that the transmission of the photo files takes too long time or the decoding of the photo files are too time-consuming.
Generally speaking, two main factors may influence image display on an image display system—one is the total processing time Ttotal of the images and the other is the quality of the images, and there is a trade-off between these two factors. When the total processing time Ttotal needs to be kept within a certain range such as a short, constant interval, the quality of the images may need to be degraded. For example, the files of the images are lossily compressed, the resolution of the images is reduced, or the parameters of the quality of the images are decreased. When the quality of the images needs to be maintained above a certain level, the total processing time of the images may need to be extended. For example, the transmitting time of large image files is increased, or the decoding time of high-quality images is prolonged. The total processing time Ttotal of an image includes image transmitting time Tt, image decoding time Td and image display time Tr. The image transmitting time Tt, from transmitting an image request till receiving an image, mainly comprises the time for transmitting a request over a network and the time for transmitting a reply image over the network, among which transmitting the reply image is the most time-consuming. When the bandwidth of the network has little variation during a short period or the network is a network with a quality of service (QoS), the most crucial factor for determining the image transmitting time Tt is the size of the image file. The image decoding time Td is a period needed for decoding the image before the image is displayed, and is corresponding to the size of the image file and the parameters of the image, e.g., formats, resolution, and compression parameters of the image. The image display time Tr is a period after the image is decoded till the image is displayed. When the total processing time Ttotal is calculated, the image display time Tr is so small, compared to the image transmitting time Tt and the image decoding time Td, that Tr can be neglected.
FIG. 1 and FIG. 2 show the timing diagrams of two conventional methods for displaying images at a constant interval. FIG. 1 shows a method using a single buffer, and FIG. 2 shows a method using double buffers. In FIG. 1, TdN represents the decoding time of the Nth image, TtN represents the transmitting time of the Nth image, and tN represents the display time point of the Nth image. Displaying images at a constant interval means that the display time point of a current image is expected to equal to the summation of the display time point of the previous image and the constant interval. Referring to FIG. 1, the summation of Tt1 and Td1 is smaller than the constant interval. That is, the transmitting time of image 1 added to the decoding time of image 1 is smaller than a predetermined constant interval, so that image 1 can be displayed at the intended display time point t1 of the image. On the contrary, the summation of Tt2 and Td2 is greater than the constant interval. That is, the transmitting time of image 2 added to the decoding time of image 2 is greater than the predetermined constant interval, so that image 2 cannot help but be displayed after the intended display time point of the image. In FIG. 2, two buffers are applied to simultaneously transmit and decode images by overlapping transmitting time and decoding time in parallel, so as to increase the efficiency of the image display system. Symbols TdN, TtN and tN in FIG. 2 are the same of those of FIG. 1, and they shall not be further described for brevity. Referring to FIG. 2, while image 0 is decoded on buffer 0, image 1 is concurrently transmitted to buffer 1. Similarly, while image 1 is decoded on buffer 1, image 2 is simultaneously transmitted to buffer 0. The method in FIG. 2 has better throughput than the method in FIG. 1; however, an additional buffer is needed for the method in FIG. 2. Furthermore, regardless of which of the methods in FIG. 1 or FIG. 2 is applied, when transmission or decoding of the image files gets too time-consuming, a result that the display time of an image exceeds a predetermined display time point is incurred. Therefore, it is an important subject of current image display field to provide a design capable of displaying images at a constant interval.